Such a device and a method for producing the device are known from US 2007/0023661 A1. The known device is arranged for detecting infrared radiation from a scene which is imaged on an array of thermally tunable filter elements. The known device further comprises a separate light source emitting light in the near infrared wavelength range. The light emitted by the light source is transformed into a collimated beam which is directed towards the array of filter elements. Since the infrared radiation from the scene is absorbed by the filter elements the temperature of the filter elements depends on the radiant flux of the infrared radiation originating from the scene. The temperature of the filter elements also affects the filter characteristic of the filter elements. In particular the transmission of the near infrared radiation through the pixel element is modified in dependency of the temperature of the filter element. By detecting the near infrared radiation transmitted through the filter elements an image of the temperature distribution and thus an image of the scene can be generated.
The known system suffers from a number of drawbacks. In praxis, the intensity of the near infrared radiation received by the detector is too low for taking pictures of a scene in a short period of time. Another disadvantage is that the filter elements are supported by thin posts, due to the required thermal isolation. In consequence, the filter elements are difficult to produce. A further disadvantage of the system is its bulkiness.
The known device can be used as an infrared (=IR) camera system. Infrared camera systems of this type may be used for various applications. For example, firefighters use infrared cameras to see through smoke, find persons, and localize hotspots of fires. With thermal imaging, power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Thermal imaging cameras are also installed in cars to aid the driver at night or in low visibility situations. Some physiological activities, particularly responses in human beings and other warm-blooded animals can also be monitored with thermographic imaging. Cooled infrared cameras can also be used in most major astronomy research activities. Military applications are also quite relevant.
The need and request for effective, sensitive, compact and relatively cheap thermal detection systems have became an important topic, in order to enable most of the mentioned applications, and to broaden their respective markets where they are already used.
Thermographic cameras can be divided into two types, those with cooled infrared image detectors and those with uncooled detectors. They differ in terms of fabrication, maintenance and performances.
Cooled detectors are contained in a vacuum-sealed cases and are cryogenically cooled, usually at 80 K—liquid nitrogen temperature—although cooling at 4 K is also possible at a much higher cost of the apparatus. The cooling increases their sensitivity since their own temperatures are much lower than that of the objects that they are suppose to measure. The drawbacks of cooled infrared cameras are that they are expensive both to produce and to maintain. Cooling and evacuating are in fact power- and time-consuming. The camera may need several minutes to cool down before starting to operate. Moreover the components capable of operating at lower temperature and pressure are generally bulky and expensive, making the whole system not easy to miniaturize in order to render it compact and portable. Despite such practical limitations, cooled infrared cameras provide superior image quality compared to uncooled ones and are thus used for applications requiring high sensitivity whereas the actual dimensions are not a problem, for instance in instruments for astronomical research. Examples of such devices include liquid helium cooled silicon bolometers, and a wide range of cheaper narrow gap semiconductor devices including indium antimonide, indium arsenide, HgCdTe, lead sulfide, lead selenide. Superconducting tunneling junction devices have been demonstrated as infrared sensors because of their very narrow gap. Further information on this type of detectors can be found, for instance, in ANGHEL et al.: Performance of cryogenic micro-bolometers and calorimeters with on-chip coolers, App. Phys. Lett. 78 (2001), 556-558.
On the other hand, uncooled thermal cameras use a sensor operating at ambient temperature, or a sensor stabilized at a temperature close to ambient using small temperature control elements like Peltier elements. An example for this type of detector can be found in US 2007/0176104. Uncooled detectors generally use sensors that detect infrared radiation by the change of resistance, voltage or current caused by the absorption of the infrared radiation. The measured variation is proportional to the intensity of absorbed radiation. Uncooled infrared sensors can be stabilized to an operating temperature to reduce image noise, but they are not cooled to low temperatures and thus do not require bulky, expensive cryogenic coolers. Such infrared cameras are therefore smaller, in some cases even portable and less costly. However, their resolution and image quality tend to be lower than the resolution and the image quality of infrared cameras with cooled detectors. Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. In particular, a microbolometer is a specific type of bolometer, composed of an absorbing grid of vanadium oxide or amorphous silicon heat sensors on top of a corresponding grid of silicon. Infrared radiation strikes the grid and changes its electrical resistance. This resistance change is measured and converted into temperatures which can be represented graphically. The sensitivity is partly limited by the thermal conductance of the pixel. Most of the commercially available thermal cameras are based on microbolometer, but they are still too expensive to be used in other applications than in military, government, high class car segments or security.
WO 2005/071770 A2 discloses a green light-emitting micro cavity Organic Light Emitting Devices (=OLED). The OLED device comprises at least one light-emitting layer, a reflector and a semitransparent reflector respectively disposed on opposite side of the light-emitting layer. Those reflectors are arranged such that the light leaving the micro cavity OLED has a substantial green spectral component.